Magnetostrictive linear actuators are well known in the art; finding extensive use in sonar applications, and more recently with deformable mirrors used in an advanced High Energy Laser (HEL) system. These devices rely on the unique properties of certain materials whose dimensions can change as a function of an applied magnetic field. To produce the desired linear movement a rod of magnetostrictive material is placed in a solenoid configuration. When current is passed through the coil, a change in the length of the magnetostrictive rod can be observed. This change in length is proportional to the magnitude of the applied field. With the newer magnetostrictive materials such as terfenol (Tb.sub.0.27 Dy.sub.0.73 Fe.sub.1.95) or an iron cobalt metallic glass such as METGLASS.RTM. 2605Co (Fe.sub.0.67 Co.sub.0.18 B.sub.0.14 Si.sub.0.01), large displacements on the order of several tens of microns can be produced. Overall performance of these actuators is comparable to currently available piezoceramic actuators, when bandwidth and hysteresis characteristics as well as displacement accuracy are considered.
These magnetostrictive actuators have several advantages over their piezoceramic counterparts, the most prominent of which is the low voltage required for operation. For airborne application, weight and power losses of high voltage, direct current power supplies required for piezoelectric actuators also impose practical constraints on system design. In addition, magnetostrictive actuators display negligible displacement drift or "creep" along the extension axis, unlike piezoceramic actuators which commonly exhibit around 5% displacement drift.
Unfortunately, magnetostrictive actuators require relatively large amounts of power either to bias the actuator at a quiescent operating point or for initialization. Consequently, they are burdened by their inherent power dissipation requirements. Depending upon the configuration, this results in an excessive amount of heat being generated near, and conducted into, the magnetostrictive rod assembly or the actuator casing. A local increase in temperature of the rod or case produces a corresponding thermal expansion in that component. The increase in length is in addition to the magnetostrictively induced change in the rod's length. The magnitude of the thermally induced change is quite significant and can substantially exceed the change in length induced magnetostrictively.
Earlier devices utilizing the magnetostrictive effect were optimized for short duty cycle AC performance. Thermal expansion of the actuator assembly was present but, not of concern. However, linear actuators of the present type require precision and stability of absolute displacement, making any uncontrolled change in length due to heating unacceptable. To date, efforts to correct for this thermal expansion have centered on passive designs to improve the cooling of the actuator assemblies themselves. However the compact nature of the electric coil and magnetostrictive rod assembly mandate inherently poor heat transfer characteristics. Moreover, their use in deformable mirror applications compound this problem, since the actuator must be located in a densely packed array.